1. Field of the Invention
This invention relates to an information recording medium, such as an optical disk capable of recording and reproducing information, a method of evaluating the reproduced signal from the information recording medium, a method of reproducing the information from the information recording medium, a method of recording information on the information recording medium, and an information reproducing apparatus.
2. Description of the Related Art
One known recordable or rewritable information recording medium is an electrothermal recording medium. This recording medium allows information to be recorded by heating and cooling an information recording medium. A typical electrothermal recording medium is a phase change medium. The phase change medium records information by making use of the difference between the phases of the medium, that is, the difference in physical properties between the amorphous and crystal line phases, such as the difference in reflectivity. For example, in an optical disk apparatus using a phase change medium, all the surface of a medium is crystallized in a prior initialization and laser light of high intensity is projected onto the medium in pulse form, thereby forming amorphous recording marks.
This is because the strong laser light melts the medium and then the medium cools rapidly and becomes amorphous, when the projection of the laser light becomes weak. On the other hand, when the information is reproduced, a weak laser light of a specific level is projected onto the medium and a change in the reflectivity caused by the amorphous sections acting as the recording marks and the crystal sections is converted into an electric signal, thereby reading the information.
An optical disk using a phase change medium recently put to practical use is a DVD-RAM (ISO/IEC16824). In the DVD-RAM, information is recorded and erased by changing the output level of laser light to be projected. The information is recorded on recording tracks of the optical disk in the form of a plurality of recording marks. The individual recording marks are formed by projecting a plurality of laser pulses onto the recording tracks. The recording waveform of the laser pulse is generally called write strategy. Write strategy, which shows a laser modulation method or a recording waveform in recording a mark on an optical disk, is defined for each length of a recording mark.
In a DVD-RAM write strategy, the optical output has three or four levels. Specifically, they are a peak power for heating a medium over the melting temperature, bias power 1 (erasing power) for holding the medium at the crystallizing temperature for the crystallization retention time, and bias power 2 and bias power 3 for cooling the melted medium rapidly to make it amorphous. In the DVD-RAM, the optical output levels are adjusted, thereby adjusting accurately the size and shape of a mark to be recorded.
In the DVD-RAM, the optical output levels defined in the write strategy are constant, regardless of the width of the mark and that of the space (no signal). Therefore, unlike a recording mark sandwiched between long spaces, a recording mark sandwiched between short spaces receives the quantity of heat for recording the preceding and following marks, which causes a thermal interference problem: the quantity of heat causes the recording mark to recrystallize or the melted portion to expand. In the DVD-RAM, to avoid this problem, the pulse width is made wider or narrower by adjusting the widths of the preceding and following spaces, thereby correcting a variation in the width of the mark and obtaining the desired shape of the mark.
On the other hand, in a method of evaluating the recorded signal (marks/spaces), asymmetry has been defined in the DVD-RAM. The asymmetry defined in the DVD-RAM is used to evaluate the difference between the reproduced waveform center level of the densest pattern of the recorded data and the reproduced waveform center level of the least dense pattern of the recorded data.
As described above, in the DVD-RAM, the write strategy is adjusted, thereby coping with a change in the recording mark area caused by thermal interference. The procedure and method for the adjustment has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-36115. In this publication, a test pattern is recorded before actual user data is recorded and the write strategy (pulse width) is adjusted. In this case, the densest pattern and the least dense pattern in the used modulation method are used as test patterns and the pulse width is adjusted so that the difference between the center levels of the reproduced signals may become zero.
As for an evaluation index of the reproduced signal in the DVD-RAM, the difference between the center level of the densest pattern and the center level of the least dense pattern is defined as asymmetry. Making the asymmetry below the standard value keeps the balance of the densest signal in the direction of amplitude with respect to the envelope of the reproduced signal, which enhances the reproducing characteristic and interchangeability of the information recording medium. When information is recorded with higher density, however, the effect of thermal interference becomes greater, which may result in a serious loss of the balance of not only the densest pattern but also other patterns in the direction of amplitude. In such a case, reproduction of the information becomes difficult and the interchangeability of the information recording medium is also lost.
Furthermore, in the DVD-RAM, although asymmetry has been defined, the procedure for adjusting the write strategy to optimize the asymmetry has not been disclosed.
In addition, jitter in the random data including the densest pattern is used as an index for determining the write strategy. However, when the recording of information gets much denser and an identification method different from a conventional slice method, such as partial response maximum likelihood (PRML), is used as the reproduced-signal processing method, it becomes difficult to measure jitter in the signal including the densest pattern. This causes the following problem: the write strategy using the jitter as an index can be determined.
Moreover, as a method of adjusting the write strategy by using asymmetry, Jpn. Pat. Appln. KOKAI Publication No. 2000-36115 has disclosed a method of recording test data about a combined pattern of the densest and least dense codes and adjusting the write strategy, particularly the pulse width, in such a manner that the center levels of the reproduced signals are aligned. However, this method cannot cope with a thermal interference problem resulting from the recording of information with higher density. Furthermore, in the correction method, the pulse width is corrected in all cases. In high-density recording affected greatly by thermal interference, the following problem arises: use of only such a correction makes it difficult to form recording marks with high accuracy and adjust the asymmetry.